The present invention relates to a wireless-type tire condition monitoring apparatus and a transmitter for the tire condition monitoring apparatus which enable checking of tire conditions, including the air pressure in a tire, from the interior of a vehicle.
Japanese Laid-Open Patent Publication No. 2001-56263 discloses a wireless-type tire condition monitoring apparatus for checking the conditions of a plurality of tires mounted on a vehicle from the interior of the vehicle. The monitoring apparatus has a plurality of transmitters respectively attached to the tires, a receiver mounted on the body of the vehicle, and a display device for informing the driver of the vehicle of the air pressure conditions of the tires. Each transmitter measures tire conditions, including the internal air pressure and the internal temperature of the corresponding tire, and transmits data indicating the measured tire conditions to an electric wave receiving device. The receiver has a plurality of receiving antennas, a synthesizer and the receiving device. The synthesizer selectively outputs to the electric wave receiving device the maximum voltages induced in each receiving antenna. Consequently, the receiver can stably receive a signal transmitted from each transmitter.
In general, the temperature in tires changes under the influence of outside air temperature, direct rays of the sun, heat generated by traveling, etc. The performance of the transmitter constituted by semiconductor electronic components depends on temperature. More specifically, the transmission power of the transmitter decreases as the internal tire temperature rises.
The transmitter rotates together with the tire when the vehicle travels. Depending on the position of the transmitter at the time of transmission, there is a possibility of the receiver being unable to receive data from the transmitter. When the transmission output of the transmitter is reduced, the reception probability, i.e., the probability of data being received by the receiver, is also reduced. According to an experiment, the intensity of the electric field at the receiving antenna decreases by 1 dBμV/m each increase of the internal tire temperature by forty degrees Centigrade.